An RFID tag uses electromagnetic induction to communicate information by supplying electrical power to an IC chip from a reading device. In order to relay information in specific frequencies with a good degree of sensitivity, the RFID tag uses resonance between its internal coil antenna and capacitor.
The coil antenna’s inductance and the capacitor’s resonance frequency need to be estimated for resonance to be produced accurately at the specified frequency. When the capacitor is external, the inductance of the coil antenna needs to be obtained accurately and the capacitor’s capacity needs to be identified. Some RFID tags have magnetic sheets or metallic films on them. The sheet’s magnetic properties and eddy currents generated in the film can affect the inductance.
This Application Note performs a magnetic field analysis of an RFID tag that has a metallic film and a magnetic sheet with a resonance frequency of 13.56 MHz, and obtains the magnetic field distribution and the RFID tag’s inductance.

The magnetic field distribution around the RFID tag is shown in fig. 1.
The magnetic field is shielded toward the metal plate, as shown in fig. 1. This magnetic field distribution has a large effect on the inductance.

The inductance of the RFID tag and the capacity of the resonant capacitor are shown in table 1.
The inductance of this RFID tag can be obtained from the relationship between the voltage and current. In this analysis, it is 942 nH. The capacity of the external capacitor, calculated from the resonance frequency and inductance, is 146 pF.